Window sash have been counterbalanced by curl springs, as explained in U.S. Pat. No. 5,353,548 to Westfall; U.S. Pat. No. 5,463,793 to Westfall, and pending U.S. application Ser. No. 11/668,112 to Tuller. The invention of this application improves on such prior art suggestions.
The counterbalancing of window sash generally requires that counterbalance force approximates sash weight. Otherwise, an overbalanced sash tends to “hop” or rise upward from an intended position, and an under balanced sash tends to “drop” or fall downward from an intended open position. Avoiding hop and drop requires manufacturers to offer counterbalance systems in a range of forces suited to a widely varying weight range of window sash. The correspondingly wide range of forces required for counterbalance systems adds to manufacturing expense.